Cathode ray tube

ABSTRACT

In a cathode ray tube comprising beam producing means to direct a plurality of electron beams from spaced apart locations to impinge on an electron receiving screen, the beams are focused on the screen by a main focusing lens which is common to all of the beams and has an optical center, and an auxiliary lens disposed between the beam producing means and the main focusing lens and having all of the beams passing therethrough with at least two of the beams being at different distances from the optical axis of the auxiliary lens, such auxiliary lens is operative to cause the beams to intersect at the optical center of the main focusing lens and has pre-focusing effects on the beams which are different at least with respect to the said two beams, and an additional focusing lens means is disposed between the main focusing lens and the screen and acts on less than all of the beams to correct for the different pre-focusing effects and thereby cause all of the beams to be focused precisely at the screen.

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0 t ilite tes ate t @avvai [54] QATHQDIE RAY TUBE [72] inventor: Masahide Sawai, Tokyo, Japan [73] Assignee: Sony Corporation, Tokyo, Japan [22] Filed: Oct. 24, 11969 [21] Appl. No.: 869,025

[30] Foreign Application Priority Date Nov. 13, 1968 Japan ..43/83l43 [52] iJ.S.Cl ..3l5/l3 [51] int. .KWHOIJ 29/50" [58] Field ofSearch ..315/13;3l3/69, 70, 77, 78

[56] References Cited UNITED STATES PATENTS 3,448,316 6/1969 Yoshida et a1. ..3 15/13 C 3,467,881 9/1969 Ohgoshi et a1. ..315/13 C Primary Examiner-Rodney D. Bennett, Jr.

Assistant Examiner-N. Moskowitz Attorney-Albert C. Johnston, Robert E. Isner, Lewis H. Eslinger and Alvin Sinderbrand [5 7] ABSTRACT In a cathode ray tube comprising beam producing means to direct a plurality of electron beams from spaced apart locations to impinge on an electron receiving screen, the beams are focused on the screen by a main focusing lens which is common to all of the beams and has an optical center. and an auxiliary lens disposed between the beam producing means and the main focusing lens and having all of the beams passing therethrough' with at least two of the beams being at different distances from the optical axis of the auxiliary lens. such auxiliary lens is operative to cause the beams to intersect at the optical center of the main focusing lens and has pre-focusing effects on the beams which are different at least with respect to the said two beams, and an additional focusing lens means is disposed between the main focusing lens and the screen and acts on less than all of the beams to correct for the different pre-focusing effects and thereby cause all of the beams to be focused precisely at the screen.

9 Claims, 13 Drawing Figures PATENTEDMARZI I972 SHEET 1 [1F 3 INVENT OR. MASAHIDE SAWAI AT TORNE Y PATENTED MAR 21 I972 SHEET 2 BF 3 m T m V m MASAHIDE SAWAI AT TORNE Y CATHODE RAY TUBE This invention generally relates to cathode ray tubes and is particularly directed to improvements in cathode ray tubes of the plural-beam type in which an auxiliary lens and main focusing lens for the plural beams are provided.

In cathode ray tubes of the plural-beam type, such as the single-gun, plural-beam cathode ray tube disclosed in US. Pat. No. 3,448,316, issued June 3, 1969, the plurality of beams emitted from the associated cathodes are refracted by means of a common auxiliary lens which pre-focuses the beams passing therethrough and which may also cause the beams to be converged substantially at the optical center of the main electrostatic focusing lens, which focuses the beams passing therethrough on the screen. Thus, the beams are made to cross each other at the optical center of the main focusing lens and then emerge from the main lens in divergent directions. Subsequently, the beams which diverge from the optical axis and from the beam lying on such axis may be deflected toward the center beam by means of convergence deflectors provided between the electron-receiving screen and the main focusing lens and spaced from the latter so that the beams land on a common area of the screen. However, in passing the plurality of beams through the auxiliary lens, at least two of the beams are at different distances from the optical axis of the auxiliary lens and the pre-focusing effect on the beams is different at least with respect to the two beams which may result in causing all of the beams to not be focused precisely at the screen. If the auxiliary lens is adjusted to compensate for the differences in the pre-focusing effect on the beams passing through the portions of the auxiliary lens at distances from the optical axis thereof, a beam passing along the optical axis of the auxiliary lens through the center portion of the lens is not precisely focused on the screen; conversely, if the auxiliary lens is adjusted to compensate for the difference in pre-focusing effect on the beam passing along the optical axis of the auxiliary lens through the center portion thereof, beams passing through the auxiliary lens at distances from the optical axis thereof are not precisely focused on the screen. Therefore, although the beams are converged at a common area of the screen, they are not all focused precisely at the screen resulting in a less desirable picture than could be present if the beams were all focused precisely at the screen.

Accordingly, it is an object of this invention to provide a new and improved unipotential, focusing type electron gun for a plural-beam cathode ray tube in which all of the beams are focused precisely at the screen.

Another object of this invention is to provide a new and improved unipotential, focusing type electron gun for a plural beam cathode ray tube in which an additional focusing lens is provided between the main focusing lens and the screen so as to cause all the beams to be focused precisely at the screen.

A still further object of this invention is to provide a new and improved unipotential, focusing type electron gun for a plural-beam cathode ray tube in which an additional focusing lens means is colocated with the convergence deflecting means so as to correct the focus of at least one of the plurality of beams in passing through the convergence deflecting means.

In a cathode ray tube according to an aspect of this invention, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of a main electrostatic focusing lens, by which the beams are all substantially focused on the electron-receiving screen of the tube, after passing through the an auxiliary lens common to all the beams, with at least two of the beams being at different distances from the optical axis of the auxiliary lens which has pre-focusing effects on the beams which are different at least with respect to the two beams; an additional focusing lens is provided between the main electrostatic focusing lens and the screen and acts on less than all of the beams to correct for the different pre-focusing effects and thereby cause all of the beams to be focused precisely at the screen.

The additional focusing lens may be colocated with the deflection means of the tube which deflects those beams emerging from the main electrostatic focusing lens along divergent paths to cause convergence of the beams at a common area of the screen.

In an additional focusing lens for a cathode ray tube as aforesaid, the lens may be constituted by a unitary annular electrode, or, if a still more effective lens is desired, by an annular electrode having a first and second annular portion at substantially the same potential and a third annular portion intermediate the first and second annular portions at a different potential than the first and second annular portions to create a focusing field therebetween.

The above, and other objects, features and advantages of this invention, will become apparent from the following detailed description of illustrative embodiments which is to be read in conjunction with the accompanying drawings, in which: a

FIG. 1 is a diagrammatic axial sectional view of an existing single-gun, plural-beam cathode ray tube;

FIGS. 2A and 2B are diagrammatic views of the optical equivalent or analogy of the electron gun of FIG. 1;

FIGS. 3A and 3B are diagrammatic views of optical equivalents of electron guns according to this invention;

FIGS. 4A and 4B are respectively enlarged diagrammatic front and side views of convergence deflecting means of an electron gun in accordance with an embodiment of the invention which corresponds to the optical analogy of FIG. 3A;

FIG. 4C is a graphical illustration of the potential distribution associated with the lens in the embodiment shown in FIGS. 4A and 4B;

FIG. 5 is an enlarged diagrammatic front view of the convergence deflecting means of an electron gun in accordance with another embodiment of the invention which corresponds to the optical analogy of FIG. 3B;

FIG. 6 is a graphical illustration of the waveform of the potential applied to the embodiments shown in FIGS. 4A and 4B and in FIG. 5;

FIGS. 7A and 7B are respectively enlarged diagrammatic front and side views of the convergence deflecting means of an electron gun in accordance with still another embodiment of the invention corresponding to the optical analogy of FIG. 3A; and

FIG. 7C is a diagrammatic axial sectional view in a horizontal plane of the embodiment shown in FIGS. 7A and 7B.

In the following detailed description of illustrative embodiments of single-gun, plural-beam cathode ray tubes according to this invention, particular reference is made to the use thereof in color picture tubes, but it is to be understood that the invention can also be applied to any other cathode ray tubes in which plural electron beams are required.

Referring to the drawings in detail, and initially to FIG. 1 thereof, it will be seen that an existing single-gun, plural-beam cathode ray tube is there shown to include electron beam generating sources constituted by three electrically separated cathodes, K K and K to which red," green" and blue video signals are respectively supplied. The three cathodes are arranged with their electron emitting surfaces in a straight line so as to be aligned with similarly arranged apertures glR, gIG and 31B in a platelike first grid G1. A second cup-shaped grid G2 has an end plate disposed adjacent grid GI and formed with three apertures g2R, 32G and g2B which are respectively aligned with apertures glR, glG and 31B.

Arranged in order following the grid G2 in the direction away from control grid G1 are successive, open-ended, tubular grids or electrodes G3, G4 and G5. Electrode G3 includes relatively small diameter end portions 3 and 4 and a larger diameter intermediate portion 5, and is supported with its end portion 3 extending into cup-shaped grid G2 and spaced radially from side wall 2 of the latter. Electrode G4 includes end portions 6 and 7 of a diameter larger than that of end portions 3 and 4 of electrode G3 and electrode G4 is mounted so that end portion 4 extends into, and is spaced radially inward from end portion 6. Electrode G5 includes an end portion 9 of a diameter smaller than that of end portion 7 and a relatively larger diameter portion 10, and electrode G5 is mounted so that its end portion 9 extends into, and is spaced radially inward from end portion 7 of electrode G4. The several electrodes G3, G4 and G5, grids G1, G2 and the cathodes K K and K are all assembled together in the above described relation by means of suitable supports (not shown) of insulating material.

In operating the electron gun of FIG. I, appropriate voltages are applied to grids G1 and G2 and to electrodes G3, G4 and G5. For example, a voltage of to 400v. is applied to the grid Gll, a voltage of 0 to 500v. is applied to the grid G2, a voltage of 13 to 20kv. is applied to the electrodes G3 and G5, and a voltage of 0 to 400v. is applied to the electrode G4, with the voltage of the cathodes as the reference. Therefore, the voltage distributions with respect to the grids and electrodes GI to G5, and their lengths and diameters are substantially identical with those of a unipotential-single beam type of electron gun which includes a first single grid member and a second grid provided with a single aperture. With the applied voltage distribution described above, an electron lens field is established between grid G2 and the end 3 of electrode G3 to form an auxiliary lens L', shown by broken lines, and an electron lens field is established around the axis of electrode G4 by the electrodes G3, G4 and G5 to form a main focusing lens L also indicated by broken lines.

In order to cause convergence of the beams B and B which emerge from electrode G5 along divergent paths, the electron gun of FIG. I further has deflecting means F that includes shielding plates P and P provided in spaced opposing relationship to each other and extending axially from the free end of electrode G5. Deflecting means F further includes converging deflector plates P, and P,', which are shown to be planar and mounted in parallel, spaced opposing relation to the outer surfaces of shielding plates P and P respectively. The plates P and P and the plates P, and P, are disposed so that the beams B B and B pass between the plates P and P between the plates P and P and between the plates P and P,, respectively. A voltage equal to that imparted to the electrode G5, such as the anode potential, is applied to the plates P and P and a voltage lower than that applied to the plates P and P by 200 to 300v. is applied to the plates P, and P,.

Thus, the three beams B B and B emanating from the cathodes K K and K are made to pass through the apertures gIR, glG and glB of grid G1 and are modulated with three different signals applied between the respective cathodes and grid G1. The beams B and 8,, pass through portions of auxiliary lens L at substantial distances from the optical axis of the latter, while beam B passes through lens L along the optical axis thereof. Auxiliary lens L is effective to converge beams B and B toward beam 8,; so that the three beams cross each other substantially at the optical center of main lens L Auxiliary lens L also exerts pre-focusing effects on beams B 8 and B However, since beams B and B5, on the one hand, and beam B on the other hand, pass through lens L at different distances from the optical axis of the latter, the pre-focusing effect on beams B and B is different from the pre-focusing effect on beam B Upon emerging from main lens L the beams B B and B pass between the plates P, and P between the plates P and P and between the plates P and P,', respectively. Since plates P and P are at the same potential, beam B is not deflected, but the beams B and 8,, which emerge from lens L along divergent paths are deflected by the electric fields established by the different potentials supplied to plates P, and P, and to plates P and P respectively, so that the three beams are made to converge or cross each other at an aperture of a beam selecting aperture grill or shadow mask G, and then diverge therefrom to impinge on red, green and blue phosphor stripes or dots arranged in sets or arrays to constitute the color screen S on a face plate of the tube. Voltages V and V applied to the electrode plates P, and P, and to the plates P and P, of convergence deflector means F are selected so that the three beams B B and B are made to cross each other at an aperture of the grill or mask G, and thus made to land only on the respective phosphor stripes or dots of an array thereof corresponding to such aperture. In this case, of course, the beams B B and B while converging at the grill or mask (3,, are substantially focused on the screen S, although not all precisely focused at the screen S due to the different pre-focusing effects of auxiliary lens L on beams B and B and on beam B respectively.

The usual horizontal and vertical deflection means, as indicated by the yoke D, are provided for horizontally and vertically scanning the three beams simultaneously with respect to the screen S as in the conventional picture tube.

Thus, by supplying red, green" and blue" color video signals between the cathodes K K and K and the grid Gll, respectively, the three beams B B and B are intensitymodulated, whereby a color picture is produced on the color screen.

Referring now to FIGS. 2A and 2B, which are diagrammatic views of the optical equivalent or analogy of the gun of FIG. I, it will be seen that the center beam B generated by cathode K passes through the center of the auxiliary lens L along the longitudinal axis of the tube, the optical axis of the lens L being coaxial with the longitudinal axis of the tube. The side beams B and B generated by cathodes K and K respectively, pass through the auxiliary lens L at portions of the latter which are spaced substantially from the optical axis of the auxiliary lens L. The beams B and B passing through the portions of the auxiliary lens L spaced from the optical axis experience different pre-focusing effects than the beam B which passes along the optical axis through the center of the auxiliary lens L.

If an attempt is made to precisely focus the center beam B at the screen, as shown in FIG. 2A, the side beams B and B, will be focused at respective points F and F B ahead of the screen at which the center beam 8 is precisely focused, thereby resulting in the side beams B and B not being precisely focused at the screen S.

If an attempt is made to precisely focus the side beams B and 8,, at the screen S instead, as shown in FIG. 2B, the center beam B will be focused at a point (not shown) beyond the screen S at which the side beams 13,, and B are precisely focused thereby resulting in the center beam B not being precisely focused at the screen S. In either case, all the beams are not precisely focused at the screen S.

In accordance with this invention, the above mentioned lack of precise focusing resulting from the differences in the pre-focusing effects of auxiliary lens L is avoided by providing additional focusing lens means located between the main focusing lens L and the screen S, as at L on FIG. 3A or as at L on FIG. 3B, which additional lens means act on less than all of the beams to correct for the different pre-focusing effects and thereby cause all of the beams to be focused precisely at the screen.

In the embodiment diagrammatically illustrated on FIG. 3A, the additional focusing lens means L is disposed to act only on central beam B and enhances the focusing effects of auxiliary lens L and main focusing lens L on such beam, whereby to reduce the distance from main lens L to the point at which beam B is focused and thereby to obtain coincidence of the points at which all of the beams are focused. Conversely, in the embodiment diagrammatically illustrated on FIG. 3B, the additional focusing lens means L act only on beams B and E and serve to increase the distance from main lens L,,, at which beams B and 8,, are brought to a focus, whereby to obtain coincidence of the points at which all of the beams are focused.

Preferably, the additional focusing lens means L or L F are colocated with the convergence deflection means of the cathode ray tube and may be disposed within such convergence deflection means. In the case where the convergence deflection means is of the electrostatic type, for example as indicated at F on FIG. 1 and at F on FIGS. 4A and 4B, the addi tional focusing lens means L represented diagrammatically on FIG. 3A may be constituted by a single tubular, openended electrode suitably mounted between plates P and P of convergence deflection means F (FIGS. 4A and 4B).

As in the convergence deflection means F of the prior art cathode ray tube (FIG. I), a high voltage V,,,, for example, the anode voltage, is applied to plates P and P and a voltage V which is lower than V for example, by 200 to 300v., is applied to outer plates P, and P,'. Further, in the convergence deflection means F, the voltage V is also applied to the tubular electrode 15 with the result that, along the axis of electrode 15, the potential varies as shown on FIG. 4C, that is, from a maximum potential at the ends of tubular electrode 15 to a minimum potential at the middle of the tubular electrode. By reason of the described potential gradient along tubular electrode 15, an electron lens field is created within electrode 15 which, when traversed by central beam 13 has an additional focusing effect on the latter. The side beams B and B passing between plates P, and P and between plates P, and P respectively, are not influenced by the field within tubular electrode 15, and thus are merely convergently deflected to impinge on a common area of the screen with central electrode B However, by reason of the additional focusing effect of the lens L constituted by the electric field within tubular electrode 15, the three beams are focused at points in a common plane corresponding to the location of the screen, as previously described with reference to FIG. 3A.

Referring now to FIG. 5, it will be seen that in a structural arrangement corresponding to the embodiment of this invention diagrammatically illustrated by FIG. 3B, the additional focusing lens means L,.-' acting on beams B and B may be respectively constituted by tubular electrodes 15A and 15B suitably mounted between plates P, and P and plates P, and P respectively, of the convergence deflection device F". In the device F", a high voltage V such as the anode voltage, is again applied to plates P and P while a relatively lower voltage V which may be 200 to 300 v. less than V is applied to plates P, and P,. In the embodiment of FIG. 5, a voltage V which is higher than the voltage Vpz is applied to tubular electrodes 15A and 1513. Thus, the potential in each oftubular electrodes 15A and 158 will be a maximum at the middle thereof and decrease to minimum values at the ends of each tubular electrode. Such potential gradient will produce an electric field forming a respective lens L (FIG. 38) having the effect on the beam B or 8,, traversing the field of increasing the distance from main focusing lens L,,, to the focus point of such beam, and thereby causing coincidence of the points of focus of the three beams, as described with reference to FIG. 3B. As shown, the high potential V may be obtained by connecting a variable DC voltage source E. between the source of voltage V and tubular electrodes 15A and 15B.

The potential applied to the outer plates P, and P,, may generally have a parabolic waveform, as shown in FIG. 6, in order to achieve dynamic convergence of the beams. If a potential having such a waveform is applied for example to the structure shown in FIG. 4A, the tubular electrode 15 will also have applied thereto a potential having a parabolic waveform so that the central beam B passing therethrough will be dynamically, as well as statically, precisely focused at the screen 8 at which the side beams B and B are precisely focused.

It should be noted that the additional focusing lens means provided according to this invention may take other forms than the unitary tubular electrode 15 or 15A and 15B of FIG. 4A and FIG. 5. Thus, for example, as shown on FIGS. 7A, 7B and 7C, if an increased focusing effect is desired, the additional focusing lens means L of FIG. 3A may be constituted by an electrode assembly 15' comprising first and second annular electrode portions 15a and 15b spaced axially apart and being coaxial with each other, and a third annular electrode portion 150 coaxial with, and extending between the first and second annular portions 15a and 15b and being of larger diameter.

In order to constitute the additional focusing lens means L,- of FIG. 3A, electrode assembly 15 has the high potential V which is applied to plates P, and P also applied to electrodes 15a and 1517, while the relatively lower voltage V, applied to plates P, and P, is also applied to intermediate electrode 15c.

Although illustrative embodiments of this invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be made therein by one skilled in the art without departing from the scope or spirit of the invention.

What is claimed is:

1. In a cathode ray tube comprising beam producing means to direct a plurality of electron beams from spaced apart locations to impinge on an electron receiving screen, main focusing lens means common to all of said beams for focusing the latter on said screen and having an optical center, and auxiliary lens means disposed between said beam producing means and said main focusing lens means and having all of said beams passing therethrough with at least two of said beams being at different distances from the optical axis of said auxiliary lens means when passing therethrough, said auxiliary lens means being operative to cause said beams to intersect at said optical center of the main focusing lens means and having pre-focusing effects on said beams which are different at least with respect to said two beams; the improvement of additional focusing lens means disposed between said main focusing lens means and said screen and acting on less than all of said at least two beams for additionally focusing each beam on which it acts to correct for said different pre-focusing effects and thereby cause all of said beams to be focused precisely at said screen.

2. A cathode ray tube according to claim 1, wherein deflection means are located between said main focusing lens means and said screen to deflect those beams which emerge from said main focusing lens means along divergent paths whereby to cause convergence of said beams at a common area of the screen, said additional focusing lens means being colocated with said deflection means.

3. A cathode ray tube according to claim 1, wherein deflection means are located between said main focusing lens means and said screen to deflect those beams which emerge from said main focusing lens means along divergent paths, said additional focusing lens means comprising annular electrode means disposed within said deflection means and having a focusing effect on at least one of the beams passing through the deflection means.

4. A cathode ray tube according to claim 3, wherein said deflection means includes a pair of spaced plates at different electrical potentials disposed at opposite sides of each of said divergent paths to electrostatically deflect the beam from the respective path, and said annular electrode means is disposed between each pair of plates along the respective divergent path and has a potential applied thereto which is different from the potentials applied to the plates to create an electric focusing field within the annular electrode means for producing said additional focusing effect.

5. A cathode ray tube according to claim 4, wherein the annular electrode means is at a higher potential than either of said potentials applied to the plates.

6. A cathode ray tube according to claim 3, wherein said deflection means includes a pair of spaced plates at different electrical potentials disposed at opposite sides of each of said divergent paths to electrostatically deflect the beam from the respective path, each of said pairs of plates, consisting of an inner plate and an outer plate considered with respect to the longitudinal axis of the tube, said inner plate being at a higher potential than said outer plate, and said annular electrode means is disposed between said inner plates along the longitudinal axis of the tube, said annular electrode means being at substantially the same potential as the outer plates to create an electric focusing field within the annular electrode for producing said additional focusing effect.

7 A cathode ray tube according to claim 6, wherein said potential applied to said outer plates and said annular electrode means has a parabolic waveform. whereby to cause static and dynamic focusing of the beam which passes between said inner plates.

8. A cathode ray tube according to claim 3, wherein said annular electrode means includes a first annular electrode portion, a second annular electrode portion spaced axially from said first annular portion and coaxial therewith, and a third annular electrode portion coaxial with said first and second annular portions and extending intermediate said first and second portions, and being of relatively larger diameter, said third annular portion being at a potential different from the potential ofsaid first and second annular portions to provide an electric focusing field therebetween.

9. A cathode ray tube according to claim 8, wherein said deflection means includes a pair of spaced plates at different electrical potentials disposed at opposite sides of each of said divergent paths to electrostatically deflect the beam from the respective path, each of said pairs of plates consisting of an inner plate and an outer plate considered with respect to the longitudinal axis of the tube, said inner plate being at a higher potential than said outer plate, and said annular electrode means is disposed between said inner plates along the longitudinal axis of the tube, said first and second annular portions being at substantially the same higher potential as the inner plates and said third annular portion being at substantially the same lower potential as the outer plates.

a at a at lOl028 0569 

1. In a cathode ray tube comprising beam producing means to direct a plurality of electron beams from spaced apart locations to impinge on an electron receiving screen, main focusing lens means common to all of said beams for focusing the latter on said screen and having an optical center, and auxiliary lens means disposed between said beam producing means and said main focusing lens means and having all of said beams passing therethrough with at least two of said beams being at different distances from the optical axis of said auxiliary lens means when passing therethrough, said auxiliary lens means being operative to cause said beams to intersect at said optical center of the main focusing lens means and having pre-focusing effects on said beams which are different at least with respect to said two beams; the improvement of additional focusing lens means disposed between said main focusing lens means and said screen and acting on less than all of said at least two beams for additionally focusing each beam on which it acts to correct for said different prefocusing effects and thereby cause all of said beams to be focused precisely at said screen.
 2. A cathode ray tube according to claim 1, wherein deflection means are located between said main focusing lens means and said screen to deflect those beams which emerge from said main focusing lens means along divergent paths whereby to cause convergence of said beams at a common area of the screen, said additional focusing lens means being colocated with said deflection means.
 3. A cathode ray tube according to claim 1, wherein deflection means are located between said main focusing lens means and said screen to deflect those beams which emerge from said main focusing lens means along divergent paths, said additional focusing lens means comprising annular electrode means disposed within said deflection means and having a focusing effect on at least one of the beams passing through the deflection means.
 4. A cathode ray tube according to claim 3, wherein said deflection means includes a pair of spaced plates at different electrical potentials disposed at opposIte sides of each of said divergent paths to electrostatically deflect the beam from the respective path, and said annular electrode means is disposed between each pair of plates along the respective divergent path and has a potential applied thereto which is different from the potentials applied to the plates to create an electric focusing field within the annular electrode means for producing said additional focusing effect.
 5. A cathode ray tube according to claim 4, wherein the annular electrode means is at a higher potential than either of said potentials applied to the plates.
 6. A cathode ray tube according to claim 3, wherein said deflection means includes a pair of spaced plates at different electrical potentials disposed at opposite sides of each of said divergent paths to electrostatically deflect the beam from the respective path, each of said pairs of plates, consisting of an inner plate and an outer plate considered with respect to the longitudinal axis of the tube, said inner plate being at a higher potential than said outer plate, and said annular electrode means is disposed between said inner plates along the longitudinal axis of the tube, said annular electrode means being at substantially the same potential as the outer plates to create an electric focusing field within the annular electrode for producing said additional focusing effect.
 7. A cathode ray tube according to claim 6, wherein said potential applied to said outer plates and said annular electrode means has a parabolic waveform, whereby to cause static and dynamic focusing of the beam which passes between said inner plates.
 8. A cathode ray tube according to claim 3, wherein said annular electrode means includes a first annular electrode portion, a second annular electrode portion spaced axially from said first annular portion and coaxial therewith, and a third annular electrode portion coaxial with said first and second annular portions and extending intermediate said first and second portions, and being of relatively larger diameter, said third annular portion being at a potential different from the potential of said first and second annular portions to provide an electric focusing field therebetween.
 9. A cathode ray tube according to claim 8, wherein said deflection means includes a pair of spaced plates at different electrical potentials disposed at opposite sides of each of said divergent paths to electrostatically deflect the beam from the respective path, each of said pairs of plates consisting of an inner plate and an outer plate considered with respect to the longitudinal axis of the tube, said inner plate being at a higher potential than said outer plate, and said annular electrode means is disposed between said inner plates along the longitudinal axis of the tube, said first and second annular portions being at substantially the same higher potential as the inner plates and said third annular portion being at substantially the same lower potential as the outer plates. 